Lecture 3: Powerpoint

COMPSCI 280 S2 2014
Enterprise Software Development
Further exploring database operations in MVC
Jim Warren, [email protected]
Today’s learning objectives

To be able to query through a variety of approaches including
sending SQL directly to the DBMS and via LINQ
To be able to write more advanced queries in the MVC
context, including joining tables and mapping the result to the
View
To be able to update the database from the MVC context
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Last lecture…

We introduced Language Integrated Query (LINQ)
using (EmployeesContext db = new EmployeesContext())
{
var emp = from e in db.Employees
where e.DateOfBirth.Year < 1975
select e;
return View(emp.ToList());
}

In this case the return value is a collection of objects of a class
we’ve already defined in the Model

Because we had said:
public DbSet<Employee> Employees { get; set; }
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and had defined the Employee class with property names exactly aligned to
the columns of the DBMS table
And in the VIEW we had said:
@model IEnumerable<MvcApplication1.Models.Employee>
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Dealing with a Join
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But what if we want to present the user with the result of a
Join on two (or more) tables?

Say that there’s a ‘role’ table which maps ‘employee’ rows by their
primary key IDnum to one or more job roles
employee

role
And I want to present the user with the meaningful fields from
SELECT * FROM employee,role WHERE employee.IDnum=role.Idnum;
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Extending the model

In your .cs file in the Models directory
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Add the new DbSet to the DbContext
public class EmployeesContext : DbContext {
public EmployeesContext() : base("MySqlConnection") { }
}
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public DbSet<Employee> Employees { get; set; }
public DbSet<EmpRole> Roles { get; set; }
And add a class definition matching the new table
[Table("role")]
public class EmpRole
{
[Key]
public int jobnum { get; set; }
public int IDnum { get; set; }
public string Role { get; set; }
}
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Fine so far… but what’s the class of the Join result?
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The Join

In HomeController.cs we can use a LINQ query
var ourEmployees =
Also perfectly good, and better use of
from e in db.Employees
the LINQ language, would be to
from r in db.Roles
replace the 2nd FROM and the WHERE
where e.IDnum == r.IDnum
with:
orderby e.Surname
join r in db.Roles on e.IDnum
select new EmpforGrid {
equals r.IDnum
IDnum = e.IDnum,
Surname = e.Surname,
GivenNames = e.GivenNames,
YearOfBirth = e.DateOfBirth.Year,
Role = r.Role};

Back in the model, we need to define this new class
public class EmpforGrid
{
public int IDnum {get; set;}
public string Surname {get; set;}
public string GivenNames {get; set;}
public int YearOfBirth { get; set;}
public string Role { get; set;}
}
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The View

In Index.cshtml (under Views/Home)
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We define the model for the view as an enumerable collection of
instances of the new class:
@model IEnumerable<MvcLetsConnect.Models.EmpforGrid>
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And we define the WebGrid itself
@{
ViewBag.Title = "People";
WebGrid grid = new WebGrid(Model);
}
<h2>People</h2>
@grid.GetHtml(columns: grid.Columns(
grid.Column("IDnum","Employee ID"),
grid.Column("Surname","Last Name"),
grid.Column("GivenNames","Given Names"),
grid.Column("YearOfBirth","Birth Year"),
grid.Column("Role","Role")))
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The result
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‘Native’ SQL

You can also send ‘native’ SQL direct to the DBMS


Create a string, not interpreted as a query by VS
And send it to the database context with the SqlQuery method
string the_name = "Tom";
nativeSQLQuery = String.Format("SELECT COUNT(*) FROM employee
WHERE GivenNames='{0}'",the_name);
var cnt = db.Database.SqlQuery<int>(nativeSQLQuery);
ViewBag.empcnt = "Count=" + cnt.First();

The SqlQuery returns a ‘generic’
class that takes a type parameter
(I’m counting here, so int is good)
I can just put anything in ViewBag and it’ll be available
Show
the
join,
a count(*)
in the
View.
Sinceand
I was counting
I only want the first
(and only) item in the collection that was returned
nativeSQLQuery = "SELECT * FROM employee,role WHERE
employee.IDnum=role.IDnum;";
var empr = db.Database.SqlQuery<EmpforGrid>(nativeSQLQuery);
return View(empr.ToList());
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Since I’ve typed this as EmpforGrid, I can use it as
the Model to pass to my View for the WebGrid
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The result
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Oh, oops! EmpforGrid has a
YearOfBirth not a DateofBirth (as in
the employee table). It forgave me
for leaving it out of the SqlQuery
result and put in a ‘convenient’
default
Easily fixed by updating the SQL:
Note I’m using the SQL syntax
Year() for the conversion, not the C#
syntax which is to invoke the .Year
method on the DateTime object
nativeSQLQuery = "SELECT *,Year(employee.DateOfBirth) as
YearOfBirth FROM employee,role WHERE employee.IDnum=role.IDnum;";
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What about the rest of the CRUD?!
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CRUD


Create, Read, Update, Delete
A CRUD matrix can be a useful specification for the scope of
programming tasks

E.g. to describe the ‘life cycle’ of each entity in a system; e.g. a hotel
reservation
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On some screen (e.g. ‘booking’) it is created (SQL INSERT, not CREATE
like making a new table)
There may be a screen to update it (e.g. ‘change booking’) which probably
also reads the current value (‘R’ of CRUD, SQL SELECT)
And there will be one or more ways to delete it (e.g. from a cancellation
action on the change booking screen or a dedicated cancellation screen,
and also once the person has checked in) – then again, you might not
actually delete in a SQL sense, but UPDATE it to cancelled or utilised
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status
Where we want to go
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And when we click an Edit link…
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The UPDATE
Native SQL version

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Given that I have a reference to an object emp of class Employee
with updated values:
Re-establish the database connection
using (EmployeesContext db = new EmployeesContext()) {
string sql=String.Format(
"UPDATE employee SET Surname='{0}',GivenNames='{1}',"+
"DateOfBirth='{2:yyyy-MM-dd}' WHERE IDnum={3}",
emp.Surname,emp.GivenNames,emp.DateOfBirth,emp.IDnum);
db.Database.ExecuteSqlCommand(sql);
...
}
Then we just .ExecuteSqlCommand (as
compared doing . SqlQuery) on the
database context to tell MySQL to have
at it
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Here we’re just building the text
of a SQL command with the help
of String.Format to plug in the
parameters from our C# code at
the curly braces (note the 3rd
parameter - #2 counting from 0! –
is formatted so MySQL
recognises the date literal
Handout 03
The UPDATE v2
Entity Framework* version
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
Now assuming a bit more about emp… not just that it’s an object
of class Employee, but that it’s already ‘attached’ to the database
context.
using (EmployeesContext db = new EmployeesContext()){
...
}

db.Entry(emp).State = EntityState.Modified;
db.SaveChanges();
The 'State' property determines
what's done by the SaveChanges
method (modified entities are
updated)
As it turns out, this will be a correct assumption once we put this
code into the appropriate part of the MVC application
*Entity Framework (EF) is an object-relational mapper that enables .NET developers to
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work with relational data using domain-specific objects –
http://msdn.microsoft.com/en-us/data/ef.aspx
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Putting the update in the MVC context

Making an Edit controller
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In HomeController.cs we need a new method to ‘catch’ the
We’ll set up the invocation such
invocation of the Edit screen
that the IDnum of the selected
record is passed to this handler
public ActionResult Edit(int id)
{
ViewBag.Message = String.Format(
"Your are editing the record for employee ID #{0}.",id);
using (EmployeesContext db = new EmployeesContext())
{
Employee emp = db.Employees.Find(id);
if (emp == null)
.Find looks up a record from the
return HttpNotFound();
dbSet based on its primary key
return View(emp);
(IDnum was set up as such in
}
MySQL and then this was
If we picked it off the WebGrid table
}
conveyed to the MVC application
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that we ourselves define it really
should be found, but good to handle
anyway (e.g. user might edit the URL
string, or somebody might’ve deleted
it since the user last refreshed their
browser screen)
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by the [Key] decorator in the
Model
Handout 03
And we need a second version…

In HomeController.cs we need a second signature for the
method
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This version catches the Edit screen when it’s being returned to us
The [HttpPost] decorator signals that this
filled out / updated by the user
is the one to receive a completed HTML
form (the previous one could’ve been
[HttpPost]
annotated as [HttpGet])
public ActionResult Edit(Employee emp) {
if (ModelState.IsValid) {
using (EmployeesContext db = new EmployeesContext()) {
db.Entry(emp).State = EntityState.Modified;
db.SaveChanges();
}
The Edit screen will post back a
return RedirectToAction("Index");
reference to the whole updated
}
Employee object, emp. If it’s valid
return View(emp);
(more on that later!) then we save
}
the update to the database
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If the data was valid, the edit is done
so redirect back to the home page;
otherwise (there’s some invalid data
on the form), present the form to the
user again
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Invoking the edit controller
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Putting an ActionLink into the WebGrid

To define the column with the Edit link on each employee:
grid.Column(header: "Edit",
format: (item) =>
Html.ActionLink("Edit", "Edit", new { id=item.IDnum }))
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OK, a bit of cryptic Razor syntax here, but note
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We’re creating an ActionLink (labelled “Edit” and that invokes the
Edit handler in our controller)
It’s per ‘item’ in the WebGrid (one link for each employee) – ‘item’
is just a magic keyword to indicate the entity on the current row
The => is a ‘lambda expression’ (more on that later… it’s a function
as a parameter)
We assign the .IDnum of the current item (i.e. it’s primary key) to
‘id’ – which is the parameter expected by our controller code!
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Inspecting
the HTML
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All that Razor
boiled down to a
hyperlink calling
/Home/Edit/2 (the
Home controller,
looking for
the Edit
method with
a parameter
value of 2)
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Where we’re up to
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
We’ve seen that we can interact with our DBMS by sending it
native SQL or using C#/.NET language-embedded syntax
(LINQ and Entity Framework)
And we can add and invoke additional handlers in our
controller (e.g. for an Edit/Update function)
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Now…
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FYI: a battery of LINQ examples:
http://code.msdn.microsoft.com/101-LINQ-Samples-3fb9811b
Work the second labsheet (if you haven’t already)
Get seriously into Assignment 2
Next lecture we’ll look at creating the HTML form to get
interactive input from the user for the Edit / Update
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